Senator Chuck Schumer, the majority leader, traveled to Ukraine on Friday for a visit meant to show American solidarity with a democratic ally under attack by Russia and increase the pressure on Republicans to drop their opposition to additional U.S. aid.

The trip, Mr. Schumer’s first official one to Ukraine, comes at a critical time, as a foreign assistance package that includes more than $60 billion in military aid for Kyiv is stalled on Capitol Hill amid Republican resistance.

It amounts to something of a victory lap for Mr. Schumer, the New York Democrat who managed to maneuver the aid bill through the Senate this month with a resoundingly bipartisan vote that came after months of partisan wrangling.

But it is also a last-ditch bid to salvage the legislation in the House. Under pressure from right-wing hard-liners hostile to funding Ukraine’s war effort, the Republican speaker, Mike Johnson, has rejected bipartisan entreaties to put it on the floor.

Mr. Schumer plans to meet in Lviv with President Volodymyr Zelensky of Ukraine and his newly appointed military commander, Oleksandr Syrsky. He said he hopes to show how congressional foot-dragging on more aid has hurt Ukraine’s efforts on the battlefield and to appeal to House Republicans to take action before it’s too late.

“We think we will be able to bring back very strong, specific evidence as to why Ukraine is, for the first time, losing the war — or, you know, retreating in the war,” Mr. Schumer said in an interview before his departure. He cited a lack of ammunition — a key component of U.S. military assistance packages since early 2022 — as a chief reason. “We also think we can bring back a view and give great detail as to how Ukraine can win this war, if they’re given what they need.”

It is unclear how loudly his message will resonate in Washington, even though Russian attacks on Ukrainian cities show no sign of abating. On Friday, the Ukrainian Air Force said it had shot down 23 of 31 Russian drones launched overnight. The drones that breached Ukrainian air defenses badly damaged residential buildings, with at least three civilians found dead under the rubble of a building in the southern port city of Odesa, local authorities said.

Mr. Schumer said the primary American audience for this trip was Republican lawmakers who had voiced support for Ukraine’s cause but voted against sending more military assistance.

That group has grown substantially over the past year, as opposition to helping Kyiv has become a political imperative on the right, fueled in part by former President Donald J. Trump’s “America First” worldview. In the fall, Republican leaders settled on a new message to justify the opposition, arguing that the United States had no business helping Ukraine maintain its sovereignty unless or until it could secure its own border with Mexico through severe immigration restrictions.

“There are a lot of people who are torn, in the Senate and the House on the Republican side, who knew this is the right thing to do but are afraid of Donald Trump’s vindictiveness,” Mr. Schumer said.

He said he believed that was the case with Mr. Johnson, though the speaker has voted repeatedly against sending aid to Ukraine.

“Speaker Johnson has talked favorably about Ukraine,” Mr. Schumer said. “I think he knows if he put a bill on the floor, it would have a majority support.”

Yet proponents of the aid have made little headway with the speaker, who has never voted to support Ukraine and is facing a threat from far-right Republicans to oust him from his position if he allows a vote on doing so.

After Representative Michael R. Turner, Republican of Ohio and the chairman of the House Intelligence Committee, led a bipartisan delegation this month to Kyiv, where he met with Mr. Zelensky and promised the United States would produce additional funding, Mr. Johnson showed no signs of changing his stance.

“The Republican-led House will not be jammed or forced into passing a foreign aid bill that was opposed by most Republican senators and does nothing to secure our own border,” Mr. Johnson said just days after Mr. Turner’s trip.

Mr. Schumer is leading an all-Democratic delegation including Senators Jack Reed of Rhode Island, the chairman of the Armed Services Committee; Richard Blumenthal of Connecticut; Michael Bennet of Colorado and Maggie Hassan of New Hampshire. Senator Mitch McConnell, the Kentucky Republican and minority leader who has been a vocal champion of aiding Ukraine, led a similarly sized G.O.P. delegation to Ukraine in 2022.

The trip represents an expansion of an international profile that Mr. Schumer has been trying to cultivate since becoming majority leader. In the past year, he has logged trips to India, Pakistan, Israel, China and various points in Europe.

“The Europeans are aghast that America — who has always been the strength of NATO, the strength of the Western alliance — might think of turning its back on Europe,” Mr. Schumer said. He added that his trip was important for “showing Europe that we’re not turning our back, we’re fighting for this and we’re going to keep fighting until we get the aid.”

Mr. Schumer added that he had no intention of succumbing to demands from some Republicans that the United States focus its efforts in Ukraine exclusively on military assistance and leave economic and humanitarian aid to European counterparts. The Senate-passed bill also includes economic and humanitarian aid for Ukraine, to help maintain civil infrastructure and assist those displaced by the fighting.

He also indicated that he and officials at the White House were exploring avenues to circumvent House Republican resistance, if their efforts failed to persuade Mr. Johnson and others to allow a vote.

“We are going to explore with the administration every way we might be able to get this aid,” Mr. Schumer said. “It’s a crucial question for the globe; it’s a crucial question for history. It goes beyond the immediate politics of the moment.”

Constant Méheut contributed reporting.